The invention relates to an improved process for fluidifying flocculated aqueous suspensions of red muds in the production of alumina by the Bayer process.
The Bayer process for the production of alumina from bauxite has long been known. It is thus unnecessary to describe it here in detail. Essentially, this technique consists first of all in placing bauxite and sodium hydroxide together, under pressure and at a given temperature, and then in decanting the liquor obtained in the presence of a flocculant (F) which is generally anionic, in order to recover the alumina, on the one hand, and a strongly basic liquor of red muds, on the other hand. As is known, these red muds must be eliminated. At the same time, the sodium hydroxide must be recovered. To do this, these liquors are decanted and washed in successive vats (two to ten according to the installations), where these liquors which have been washed by the preceding liquor are stirred and recycled counter-current. The sodium hydroxide concentration is thus gradually brought from an initial content of 200 to 300 g/liter to a final content in the region of ten grams per liter. Simultaneously, the concentration of red muds is gradually brought within the region of 40 to 50% dry solids content. However, the presence in these liquors of the flocculant (F) increases the concentration of red muds, which then become increasingly difficult to convey by pumping to the elimination site, often referred to as the "lake" or "lagoon".
In the document CA-A-819,040, corresponding to the document DE-A-1,767,422, in order to facilitate the decantation of these highly viscous suspensions it has been proposed to add to the decantation vats a flocculant mixture comprising starch and a copolymer of acrylamide and sodium acrylate of molecular weight greater than 50,000, preferably of the order of ten million. This technique, which favors the decantation, unfortunately increases the viscosity of the suspensions, which then become more difficult to pump.
In the document U.S. Pat. No. 4,169,824, corresponding to the document FR-A-2,430,792, it has been proposed, after the final washing step, to add to the liquor of flocculated red muds which is intended to be eliminated, a dispersing agent (D) based on an acrylic acid polymer, in particular a methyl methacrylate polymer, of molecular weight between 5,000 and 30,000, and this in a proportion of 50 grams per tonne (50 g/t) of dry red muds. In this way, the viscosity of the final solution of the muds is lowered, the latter becoming more fluid and thus able to be pumped in existing installations, and consequently conveyed to the point of final elimination. Although this approach gives excellent results, it remains expensive to implement on the whole, because it is necessary to use appreciable amounts of dispersing agent (D). In addition, this approach does not resolve the problem of the decantation itself, or the poor flow of these viscous muds.
Moreover, the injection of a dispersing agent (D) immediately after the final washing, but before the pump, requires the use of a specific additional dispersion material, and requires the evacuation tubing and pumps to be reinforced, which increases the investment cost. In addition, for a given concentration of muds, this decreases the pumping efficiency. Finally, the mixing being carried out in the connecting tubing just before the pump is performed in a very short space of time, which is far from being ideal in order to ensure a good homogeneity of the medium to be pumped.
In document U.S. Pat. No. 5,043,077, it has been suggested to add a mixture containing leonardite to the final dilution vat. This diluent is added in an appreciable amount (from 0.1 to 10% relative to the amount of dry muds). This requires the addition of the suitable material, and consequently increases the investment cost. Since a solid material is added, the latter will affect the subsequent pumping efficiency. Finally, the use of this very specific diluent sometimes brings about precipitation by agglomeration around this compound. This is no doubt the reason for which this technique has barely been developed.
In order to accelerate this decantation, it has been suggested in the document GB-A-2,080,272 to use a mixture of two flocculants (F), introduced in series preferably in the latter stages of the treatment, namely an emulsion of sodium polyacrylate and a polyacrylamide, both of molecular weight between five and ten million.
This process also has the same disadvantages as the above processes in which a single flocculant is used, in particular as regards the pumping difficulties.